The present invention relates to dynamoelectric machines and, more particularly, to a keybar structure for interfacing a laminated core of a dynamoelectric machine to the supporting structural frame.
Large dynamoelectric machines such as, for example, large electric generators, employ cylindrical stator cores composed of stacked laminations of thin layers of a magnetic material. Conventionally, the stator core is built up within a cylindrical frame using dovetail keybars attached to the stator frame upon which dovetail slots in lamination segments are placed. When the entire stator core is completed, axial forces are applied to the ends of the stator core to retain the stator core as an essentially unitary mass.
In producing the stator core, a plurality of annular web plates are first aligned within a cylindrical outer wrapper plate. Then a set of, for example, 12 to 18 key bars are fitted each within axially aligned slots in the inner periphery of the annular web plates. Each key bar includes a dovetail extending radially therefrom toward the center of the stator core. Each lamination segment includes a plurality of equally spaced dovetail slots fittable upon the keybar dovetails.
The magnetic forces imposed on the stator core of a large dynamoelectric machine tend to deform the stator core into a slightly oval cross section. As the rotor of the large dynamoelectric machine rotates at a speed of, for example, 3600 RPM, the oval deformation rotates with it. Each point about the stator core thus experiences cyclic radial forces at a frequency of twice the rotational frequency of the rotor. A close fit between the keybar dovetails and the lamination dovetail slots is essential to avoid relative vibration of the laminations and keybars and the production of an annoying buzzing noise at twice the rotational frequency.
In a structure as large as the stator of a large dynamoelectric machine, it is difficult to establish and to maintain the level of precision required to permit accurate stacking of precisely fitting lamination dovetail slots onto keybar dovetails. The conventional approach to positioning the keybars includes initially providing oversize rectangular slots in the annular web plates into which rectangular keybars are placed. The keybars are then blocked into position and are affixed to the annular web plates using bridging U-rings which are welded to the keybars and to the annular web plates, effectively bridging the gaps produced by the oversize slots. Maintaining such alignment during the step of affixing the U-rings is complicated by the distortions produced by the relatively heavy welding which is employed. If a keybar misalignment prevents fitting the lamination dovetail slots thereon, it may be necessary to file or grind an edge of the dovetail to permit assembly. Such grinding may decrease the tightness of fit and thus contribute to vibration. An improved technique for providing more accurate alignment of the keybars is therefore desirable.
Substantial mechanical forces are imposed at the interface of the keybars and the annular web plates. The conventional use of rectangular oversize slots in the annular web plates may permit the development of stress concentrations, especially at the corners of the oversize slots which may contribute to crack initiation. A technique for reducing the possibility of stress concentrations at the interface between the keybars and the annular web plates is also desirable.